1. Field of the Invention
The present invention relates to a plasma arc power supply including a plurality of power supply units used in a welding machine or a cutting machine and a control method for the plasma arc power supply.
2. Description of the Related Art
A power supply for a plasma arc used in plasma cutting or plasma welding is required to be compact and lightweight and is formed by a direct current (DC) power supply including an inverter. When a large output is required, a plasma arc power supply may include a plurality of DC power supplies that are driven in parallel.
Each of the DC power supplies first rectifies and smoothes an input alternating current (AC), which is for example commercial power, thereby converting the AC to a DC. Each DC power supply next converts the DC to a high-frequency AC using a high-frequency inverter circuit, which is formed by a semiconductor switching element, such as an IGBT (insulated gate bipolar transistor), a MOSFET (metal oxide semiconductor field-effect transistor), and a bipolar transistor. Each DC power supply rectifies and smoothes the high-frequency AC using a transformer to generate a DC output. Finally, each DC power supply supplies the DC output to a torch and a base material as loads. The operating frequency of the switching element of such a DC power supply is controlled based on feedback using an error signal indicating a difference between an output current detection signal and a reference signal. Such feedback control enables the DC power supply to supply a constant current to the load.
To start an arc, a pilot arc with a small current is first formed between a nozzle electrode and a main electrode of the torch. The torch then moves toward the base material to cause shifting from the pilot arc to a plasma arc (main arc) with a large current formed between the main electrode and the base material.
To form a pilot arc, the plasma arc power supply may include a DC power supply for a pilot arc in addition to the main power supplies. The DC power supply for a pilot arc may have a negative terminal connected to the main electrode and a positive terminal connected to the nozzle electrode (see, for example, Japanese Unexamined Patent Publication No. H11-254144).
However, the DC power supply for a pilot arc, which is arranged in addition to the main power supplies as described in Japanese Unexamined Patent Publication No. H11-254144, increases the number of components and complicates the structure of the plasma arc power supply, and consequently increases the cost of the entire plasma arc power supply.